The invention relates to a circuit arrangement for operating discharge lamps. Of particular interest here are circuit arrangements containing a power factor correction device and an inverter. The invention relates to the coupling of the power factor correction device and the inverter.
Electronic operating equipment for discharge lamps operated using a mains voltage should only draw a mains current which satisfies the relevant standards. For example, the standard IEC 61000-3-2 sets limits for the amplitudes of the harmonics of the mains current. Electronic operating equipment which contains circuit arrangements having a separate power factor correction device so that they meet the standards for mains current are in widespread use.
The power factor correction device generates an intermediate circuit voltage which supplies power to an inverter. The inverter generates a radiofrequency a.c. voltage which supplies power to the discharge lamps. Radiofrequency here is understood as a.c. voltages having a frequency which is considerably higher than a frequency of the mains voltage.
The power factor correction device and the inverter contain electronic switches which are switched on and off. This causes the power factor correction device and the inverter to oscillate, the power factor correction device oscillating with a power factor correction clock cycle and the inverter oscillating with an inverter clock cycle.
The oscillations of the power factor correction device and the inverter must be started in a targeted manner: A problem arises when starting up the circuit arrangement. When starting up the circuit arrangement, the mains voltage charges a storage capacitor to the peak mains voltage value. This results in high mains current values which can cause interference in the power factor correction device. It is therefore important not to start the oscillation of the power factor correction device until the charging process of the storage capacitor is complete when starting up the circuit arrangement.
Once the power factor correction device has started oscillating, a controlled value for the intermediate circuit voltage is set across the storage capacitor. If the intermediate circuit voltage value, despite being regulated, exceeds a predetermined overvoltage threshold, an overvoltage shutdown must take place. During the overvoltage shutdown, the oscillation of the power factor correction device is interrupted in order to protect components of the circuit arrangement from an overvoltage.
A further problem arises in the event of a fault. A fault may occur if the discharge lamp has reached the end of its life, is defective or is not present. A fault may also occur if an attempt is made to operate a lamp which is unsuitable for the circuit arrangement. In the event of a fault, a fault shutdown must take place which stops the oscillation of the inverter. The fault shutdown protects components of the circuit arrangement from an overload.
If a fault shutdown has occurred, the inverter no longer draws any power from the power factor correction device. It is advantageous that, in the event of a fault shutdown, the oscillation of the power factor correction device is also stopped. The operating equipment as a whole is thus transferred to a shutdown mode which does not draw any power from the mains voltage and has a minimum load on the components.
Circuit arrangements which contain control circuits controlling the oscillation of power factor correction devices and inverters are known. These control circuits increase the complexity, and thereby the costs, of the circuit arrangements.
The object of the present invention is to provide a circuit arrangement for operating discharge lamps, which provides cost-effective control of the oscillation of the power factor correction device and the inverter.
The invention is based on a circuit arrangement which does not contain a control circuit controlling the oscillation of the power factor correction device and the inverter. Rather, the power factor correction device and the inverter may oscillate independently of one another. The oscillation of the power factor correction device is started according to the invention, by means of a starting device, by an oscillation of the inverter.
When starting up the circuit arrangement, the oscillation of the power factor correction device does not start automatically. It is only when the inverter begins oscillating that the oscillation of the power factor correction device is started by means of the starting device. Since the inverter can only begin oscillating when the charging process of the storage capacitor is largely complete, because only then is an intermediate circuit voltage available, no problems arise owing to high mains current values when starting up the circuit arrangement.
The power factor correction device advantageously contains an overvoltage shutdown which stops the oscillation of the power factor correction device when the intermediate circuit voltage exceeds a predetermined overvoltage threshold.
The overvoltage shutdown advantageously operates in a monostable manner. This means that once the overvoltage shutdown has responded, the oscillation of the power factor correction device stops for a given shutdown time. Once the shutdown time has elapsed, the overvoltage shutdown becomes inactive again and the oscillation of the power factor correction device can be started again by means of the starting device. The duration of the shutdown time depends primarily on the decay behavior of the oscillation of the power factor correction device. It must be ensured that the shutdown time lasts until the oscillation of the power factor correction device has decayed. In practice, the value of the shutdown time is at least 100 microseconds.
The inverter advantageously contains a fault shutdown which stops the oscillation of the inverter in the event of a fault.
It is advantageous when the fault shutdown operates in a bistable manner and the overvoltage shutdown operates in a monostable manner. The oscillation of the power factor correction device and of the inverter therefore influence one another as follows: If the overvoltage shutdown responds without a fault being present then, although the oscillation of the power factor correction device is stopped, the inverter continues to function and operates the discharge lamps. Once the shutdown time has elapsed, the power factor correction device is started again. In the event of a fault, the oscillation of the inverter is stopped permanently. Since power is no longer drawn from the power factor correction device, the intermediate circuit voltage increases until the overvoltage shutdown responds. Even after the shutdown time has elapsed, in this case the oscillation of the power factor correction device is no longer started, since the oscillation of the inverter has stopped owing to the fault shutdown. According to the invention, the circuit arrangement thus goes into a shutdown mode in the event of a fault, without a complex control circuit.
For cost reasons, it is advantageous to design the power factor correction device as a self-oscillating step-up converter having a step-up converter switch, a step-up converter inductor and a step-up converter diode. Such a step-up converter is described in the German patent application having the official file reference 10205516.5 of Feb. 8, 2002. With this step-up converter, the voltage across the step-up converter switch forms a feedback variable. A feedback loop which is in principle capable of oscillating is thus closed. According to the invention, however, the step-up converter is dimensioned such that it does not automatically start an oscillation.
The step-up converter is only started, by means of a starting device, by the oscillation of the inverter. According to the invention, the starting device is realized by a trigger capacitor, by means of which the oscillation of the inverter is superimposed on the feedback variable. The oscillation of the inverter thus triggers a first oscillation of the step-up converter, whereupon the latter continues to oscillate automatically. The value of the feedback variable during oscillation is large enough that the starting device can exert no influence on it.
For cost reasons, it is advantageous to design the inverter as a half-bridge inverter. Particularly cost-effective are known self-oscillating half-bridge inverters.
A half-bridge inverter provides a radiofrequency a.c. voltage at its output. The tie point of the two half-bridge switches of the half-bridge inverter forms its output. There, the oscillation of the inverter can be tapped off for the starting device.
A first terminal of the trigger capacitor is accordingly coupled to the tie point of the two half-bridge switches. When using a self-oscillating step-up converter, a second terminal of the trigger capacitor is coupled, according to the invention, to the feedback variable of the self-oscillating step-up converter so that an oscillation of the step-up converter is triggered. Therefore, the second terminal of the trigger capacitor is coupled, according to the invention, to the tie point of the step-up converter inductor and the step-up converter diode.